Since data are recorded on an optical disk with dense minute marks, an optical disk drive must use an optical pick-up head to project laser light focused on the marks and convert the magnitude of the light reflected by the marks into a digital signal which is then decoded into a data signal. Therefore, the accuracy of focus may directly influence the magnitude of the received signal and the correctness of data.
As shown in FIG. 1, a conventional method for calibrating focus balance in an optical drive which employs a pick-up head 1 to project a light beam 2 onto a optical disk 3, and the optical disk 3 reflects the light beam 2 onto a photodetector 4 through the pick-up head 1. The photodetector 4 has four equally divided light receiving sections A, B, C, and D, each of which receives a different portion of the reflected light beam and converts the received light into an electrical signal of corresponding magnitude. The electrical signal is then input into an amplifier 5. The electrical signals from the light receiving sections A and C and the light receiving sections B and D are added respectively to form the electrical signals (A+C) and (B+D), and then the electrical signal (B+D) is subtracted from the electrical signal (A+C) to form a differential signal. The differential signal is then amplified to form a focus error signal FE (Focus Error) and transferred to a compensator 6. The compensator 6 generates a control signal, which controls and calibrates the pick-up head through a focus servo unit 7 to lock the focus spot on the mark of the fast spinning and vibrating optical disk 3, such that the four light receiving sections of the photodetector 4 are able to accurately receive the reflected light. An amplifier 8 adds the electrical signals (A+B+C+D) to form a radio frequency signal RF (Radio Frequency) and uses the signal representing the mark to enhance the reliability of the output data signal from the radio frequency signal RF modulated by the modulator 9.
However, since the focus spot is a small region of short distance, the aforementioned focus spot locked by focus calibration might not be the location of the strongest radio frequency signal RF. Also, the aforementioned focus calibration employs a small electrical signal as means for determination, which is susceptible to the errors of the disk drive itself such as the non-uniformity of the light beam projected by the laser light source, the precision of the optical system, or the heterogeneity of the light receiving materials forming the four light receiving sections of the photodetector 4, leading to difficulty in ensuring locking of the focus spot. As a result, there is another conventional apparatus for calibrating focus balance in an optical disk drive, in which the amplifier 8 outputs a radio frequency signal RF and the radio frequency ripple circuit (RFRP), i.e. RFRP circuit 10, is used to convert the radio frequency signal RF into a waveform of the RFRP signal. The focus balance unit 11 records and compares several sequential RFRP signals to find out the strongest RFRP signal, and then generates an error signal. The compensator 6 generates a control signal and locks the focus spot range in combination with a focus error signal FE. The focus servo unit 7 controls and calibrates the pick-up head 1 to achieve the focus condition of the strongest RFRP signal, which directly ensures the radio frequency RF used by the pick-up head 1 for reading the marks is the strongest in order to facilitate decoding.
However, the single radio frequency signal RF only represents a mark of binary code 1 or 0, which still needs modulation and cannot be directly output by the optical disk drive. Taking the conventional EFM (Eight to Fourteen Modulation) for example, the original 8-bit digital signal is modulated into a 14-bit signal to form a mark on the optical disk in the binary digit form of 1 or 0. As a result, the pick-up head must completely receive the 14-bit radio frequency signal RF one by one before being accurately demodulated into an 8-bit digital signal by the modulator 9. Making the radio frequency signal RF of a single mark the strongest does not guarantee that the other marks in the same group of digital signals are also the strongest. Sometimes, in the course of seeking the strongest radio frequency signal RF, the focus calibration tends to result in obscurity of some marks in a group of digital signals and leads to failure in demodulation. Therefore, the method for seeking the strongest radio frequency signal RF still fails to effectively enhance the whole performance of the optical disk drive. Accordingly, there are still problems to be solved in the conventional method for calibrating focus balance in a disk drive.